It's the solo-tradeoff you get per role.
DPS? You kill stuff faster, but can't tank much. Better kill the enemy before it kills you. Don't be too ambitious with aoe'ing stuff when you're on your own. (even 2 enemies can be risky depending on level / gear / enemy level)
Tank? Can tank enemies without much trouble, though HP isn't endless (gets some self healing in later levels depending on class). Less damage, so takes longer to take down stuff - but can afford to aoe a few mobs at a time (may still run out of HP though if its too many).
Healer? Won't ever run out of HP (..unless out of Mana first). Also less damage - actually not sure if heal or tank deal less. Can afford to aoe a bunch of mobs (once aoe skill is unlocked around lv45+), but gotta be careful cause not very tanky. So the heals need to be able to keep up with the incoming damage while able to cast their aoe in-between still.
So each role has its advantages and disadvantages in solo play. I'd say DPS has the biggest risk factor out of the three, you easily end up dead if you're careless - but on the other hand get to kill stuff the fastest out of the three.